The synchrotron radiation from the Wisconsin 240 MeV electron storage ring was used to calibrate the absolute sensitivity of photoelectric detectors to radiant flux at wavelengths between 1100 and 3000 Å. The spectrum from an electron in the storage ring beam was theoretically predicted and the number of electrons in the beam determined exactly by observing the discrete decreases in radiant flux as single electrons were removed from the stored beam. Interference filters of measured transmission were used to limit the flux to the particular wavelength bandpasses desired (200–300 Å). Photomultipliers for use as laboratory standards of radiant flux, and more recently flight photometers for stellar observation via rockets, were calibrated against this standard.